1. Field of the Invention
The present invention relates to a drive controlling apparatus suitable for use as a lens controlling apparatus for a camera or the like.
2. Description of the Related Art
The recent development of video instruments such as video cameras, electronic still cameras and camera-integrated VTRs is remarkable. In particular, the functions and operability of such video instruments have been greatly improved and their size and weight have been increasingly reduced. Among others, camera-integrated VTRs have been rapidly gaining in popularity, and great reductions in their size and weight have been realized owing to the minimization of the number of parts used per VTR as well as changes in the structures of the VTRs themselves.
For example, such a camera-integrated VTR includes a lens unit which requires relatively large space and parts.
FIG. 1 shows one example of a structure of a so-called inner focus type. The inner focus structure is known as an arrangement in which a front lens element is fixed in position and rear lens elements are used to vary magnification or to adjust focus, whereby the size of a lens unit can be minimized.
The lens unit shown in FIG. 1 includes a fixed front lens 101, a magnification varying lens (zooming lens) 102, an iris 103, a fixed third lens 104, and a fourth lens (focusing lens) 105 which performs a focusing function and the function (compensator function) of correcting the movement of a focal plane due to the movement of the zooming lens 102.
As magnification is varied by moving the zooming lens 102 in the lens unit arranged as shown in FIG. 1, the fourth lens 105 operates to perform the compensator function and the focusing function as described above. The manner of this operation is shown in FIG. 2.
FIG. 2 shows the positional relation between the zooming lens and the focusing lens with a subject distance as a parameter, and the horizontal axis represents the position of the zooming lens, while the vertical axis represents the position of the focusing lens. As is apparent from FIG. 2, during zooming, if the focusing lens moves along a locus unique to each subject distance, it is possible to continue the zooming without defocus, i.e., in an in-focus state. If the movement of the focusing lens deviates from the unique locus, defocus will occur.
A method of moving the focusing lens along a locus unique to each subject distance during zooming is proposed in, for example, Japanese Laid-open Patent Application No. Hei 1-280709. In this method, the loci of the focusing lens having the focusing function and the correcting function (compensator function) of correcting the movement of a focal plane due to the movement of the zooming lens shown in FIG. 2 are divided into zones each including a group of loci drawn at an approximately equal inclination, as shown in FIG. 3, and one speed is assigned to each of the zones as a representative speed. During zooming, any one of the zones is selected on the basis of the positional relation between the zooming lens and the focusing lens, and while both lenses are positioned within the selected zone, the focusing lens is made to move at the representative speed assigned to the zone.
However, the above-described method has the problem that the representative speed for each of the zones is determined with respect to a single zooming-lens moving speed and if the zooming-lens moving speed varies due to, for example, a variation in a zooming-motor output, a temperature change, a change in the attitude of the lens unit due to a change in a camera angle or the like, the focusing lens does not correctly follow the loci of FIG. 2.
Japanese Laid-open Patent Application No. Hei 1-319717 proposes a method of adjusting a zooming-lens driving speed during zooming by increasing or decreasing a coefficient to be multiplied by the aforesaid representative speed in accordance with a change in an actual zooming speed.
Referring to FIG. 3, for example, the horizontal axis is divided into 16 equal parts. If it is assumed that a design zooming speed is set to a speed which permits the zooming lens to move between a telephoto end (T) and a wide-angle end (W) in 7 seconds, 26 vertical sync periods (26 V sync) are required for the zooming lens to pass through a single zone 401 as shown in FIG. 4 in the case of the NTSC system. If N [V sync] is taken to pass through the single zone during actual zooming, the change ratio RZS of the actual zooming speed to a reference value (T.revreaction.W: 7 sec) of the zooming speed is expressed as: EQU RZS=N/26 (1)
Accordingly, during zooming, by always measuring the number of vertical sync periods required to pass through the aforesaid single zone and multiplying 1/RZS by the aforesaid representative speed, it is possible to perform the zooming at a focusing-lens moving speed according to a variation of the zooming speed without defocus.
However, the aforesaid example has the following disadvantages since the measurement of the zooming speed or calculations on Equation (1) have been performed by a microcomputer.
(i) If measured values or measurement results are stored in a volatile memory such as a RAM, the stored data are lost when a power source is turned off, and are not used for later control.
(ii) To compensate for the disadvantage (i), data may be stored in a non-volatile memory such as an E.sup.2 PROM. However, if the lens unit is not used for a long time or an environment or the aforesaid attitude changes when the power source is again turned on, the zooming speed may change, causing zooming to start at an erroneous focusing-lens driving speed.
(iii) In association with the disadvantage (i), if zooming is initially performed with data lost after the power source has been turned on, focusing control does not respond to the zooming until a stable measured value N is obtained, and the zooming may start at an utterly different focusing-lens speed.
In a lens position detecting system utilizing the above-described example, if a variable-resistance type of encoder is used as, for example, a zooming-position detector, the following drawbacks will arise. As shown by 501 in FIG. 5, the state of a change in the resistance of the encoder with respect to the angle of rotation thereof may vary, depending on the angular position of the encoder. Otherwise, as shown by 502, a monotonic increase may be partially impaired and an irregular variation may occur.
If boundaries are provided in the output value of the zooming encoder to divide the entire zooming movement range into zones as shown in FIG. 3, the zones relative to the position of the zooming lens show a characteristic such as that shown in FIG. 6. The portion 601 of FIG. 6 has a zone length longer than a desired zone length due to the influence of the non-linear portion 501 of FIG. 5, whereas the zone value of the portion 602 of FIG. 6 undergoes chattering by the influence of the non-monotonic increase shown by 502 in FIG. 5.
For example, if the measurement of the zooming speed is performed by the above-described method, it will be determined that the speed measured at the portion 601 is slower than an actual zooming speed and that the speed measured at the portion 602 is far faster than the actual zooming speed. If such a measurement result is, as it is, applied to the focusing-lens moving speed during zooming, defocus will occur in the part of an image which corresponds to the portion 601 or 602.
To cope with the above-described disadvantage, an arrangement may be considered in which the speed of the zooming lens is actually measured and if the aforesaid abnormal measurement data is obtained, the data is not used in order to prevent abnormal follow-up operation of the focusing lens. However, for example, if zooming is initially performed after the power source of the apparatus is turned on, since no zooming speed has been measured, the above-described ratio RZS is not determined from the moment the first zooming starts until the moment the first zooming speed is completely measured. As a result, the moving speed of the focusing lens is not appropriately controlled and defocus may take place.